Anterior cruciate ligament (ACL) injuries occur in as many as 250,000 Americans each year. Within 10 years, 1 in 3 individuals who sustain an ACL injury develop signs of post-traumatic knee osteoarthritis (OA), regardless of whether or not they underwent ACL reconstruction. Radiographic evidence of knee OA rises to 52% in ACL + Meniscus injured patients at 12 years post-injury. Prior history of ACL injury increases an individual’s ACL injury risk by 4-6 times to both the previously injured and healthy opposite limb.

It is unknown if other, more common and less severe lower extremity injuries are associated with an elevated risk of ACL injury. Understanding the association between prior lower extremity injuries and incident ACL injury may help in identifying those with elevated risk for future ACL injury and post-traumatic knee OA. Individuals with a history of more common lower extremity injuries may be important to target for ACL injury and ultimately post-traumatic knee OA prevention efforts. The objective of this study was to determine if history of specific lower extremity injuries are prospective risk factors for incident ACL injury.

What Did You Do and Find In This Study?

First year cadets from the U.S. Military, U.S. Air Force or U.S. Naval Academies were enrolled in the JUMP-ACL study (Joint Undertaking to Monitor and Prevent ACL Injury) as participants as they began their 4-year officer training. A total of 5,689 healthy, physically active first year cadets were enrolled between 2005 to 2009 (Males = 3,496; Females = 2,193; Age = 18.6 ± 0.6 yrs; Height = 173.5 ± 9.2 cm; Weight = 71.9 ± 12.9 kg).

Participants completed a self-reported injury history questionnaire on prior injuries to the lower extremity over the past 6 months. History of the following lower extremity injuries was obtained:

Ankle sprains

Shin splints (medial tibial stress syndrome)

Knee swelling / popping

Knee meniscus injury

Patellofemoral pain

Hip injury

Active surveillance was conducted for all participants with no prior history of ACL reconstruction at study baseline as participants were prospectively followed for 4 years. ACL injuries were ascertained through medical record abstraction at each academy and through searches of records in the Defense Medical Surveillance System, a comprehensive database of military encounters by the Armed Forces Health Surveillance Center. ACL injury was confirmed intra-operatively for 98% of injuries.

117 of the 5,689 individuals with no prior history of ACL reconstruction were later diagnosed with an incident ACL injury during the follow-up period (74 male and 43 female). Among the primary ACL injuries, 60 (51%) were non-contact, 38 were indirect contact (32%), and 19 (16%) were direct contact. The 4-year risk of ACL injury was 2.0% (95% CI = 1.7%, 2.4%).

The rate of ACL injury was the number of incident ACL injuries divided by total person-years at risk. Rate ratios (RR) were calculated using multivariate Poisson regression models (adjusted for gender, testing site, and year of enrollment).

Ankle sprain history was the most important predictor of ACL injury. These findings suggest that history of an ankle sprain could be used by health care providers to target ACL injury prevention efforts. Interestingly, prior injury to the hip, knee and shin were not associated with future ACL injury risk. Ankle sprain events may be similar to ACL injury events in that an unexpected perturbation occurs where the neuromuscular system is unable to mitigate the injurious external loads.

Because ACL injury is most common in people between the ages of 15-24 years, these individuals develop knee OA very early in life adding to a myriad of inactivity related physical and psychological co-morbidities. Targeting individuals with a history of ankle sprain for ACL injury prevention efforts may prevent incident ACL injury and subsequently reduce the risk of knee OA development.